The present invention relates to a combined electrostatic objective and emission lens system for a particle-optical instrument, such as a microbeam probe for SIMS, which system includes a plurality of electrodes, each coupled to a respective source of potential and, in cooperation with a conductive sample surface, focuses a bundle of primary rays on a small spot of the sample surface and collimates the secondary particles emitted by this spot into a bundle of secondary rays which leaves the electrode system in a direction substantially opposite to the bundle of primary rays.
Methods of investigating surfaces and solids with the aid of charged particles (electrons, ions) are acquiring increasing importance, in particular in the fields of materials research, metallography, solid-state physics, semiconductor technology, geochemistry, biochemistry and protection of the environment.
One of these methods is secondary ion mass spectroscopy (SIMS), which is distinguished by particularly high sensitivity, applicability to all elements with simultaneous isotope separation and the possibility of microanalysis.
In the investigation of a narrowly limited surface area of a sample (microanalysis) by SIMS, a primary beam is precisely focused on the surface of the sample by means of an electrostatic objective lens. Arrangements in which the secondary ions produced are carried out backwards through the objective lens prove to be particularly advantageous with a view to the attainment of the smallest primary beam diameters in association with high current density. A microbeam probe of this type is known from Federal Republic of Germany Published Patent Application No. 22 23 367, published Nov. 11, 1973.
The above-mentioned microbeam probe contains an objective with two serially arranged electrostatic lenses with rotation symmetry and of short focal length, and a diaphragm arranged between them. The surface to be examined is electrically conductive. The lenses and the surface to be examined are so arranged and biased as regards the energy of the bundle of primary rays that this bundle of rays is focused on the area of the sample by the combined action of the electric fields of the two lenses, the diaphragm acting as a stop for the bundle of primary rays and the secondary particles produced in the area of the sample being focused at the same time into the opening of the diaphragm by the lens formed by the electrodes of the second lens and the conductive surface. Between the primary ray source and the objective there is disposed an arrangement for producing a deflecting field which separates the bundle of primary rays and the bundle of secondary rays by reason of the different energies of the particles of these bundles. In this known microbeam probe, the electrostatic lens adjacent the surface of the sample must have a small focal length, so that in the known construction only small field strengths are admissible at the surface of the sample. This is disadvantageous, since in an electrostatic emission lens the virtual (apparent) diameter of an emitting point is proportional to the quotient of the initial energy of the secondary particles to the field strength at the surface. The higher the field strength at the surface, the better is the emittance of the bundle of secondary rays, i.e. the product of the virtual diameter and the angle of aperture of the bundle of secondary rays starting from this point.
It is true that from Federal Republic of Germany Published Patent Application No. 28 42 527, published Apr. 3, 1980, an electrostatic emission lens is already known wherein the field strength at the electrically conductive surface of the sample is limited, practically speaking, only by the breakdown field-strength of the section between the surface of the sample and the first lens electrode following on this surface. This known emission lens is not, however, employed as an objective lens for focusing the primary beam, but on the contrary, the latter is introduced laterally at a relatively large angle to the axis of the emission lens through additional apertures in the electrodes.
The most important disadvantage of the above-mentioned known lenses consists, however, in that the primary particles and the secondary particles must have charges of opposite signs.